1. Field of the Invention
The invention concerns opto-electronic systems used for optical transmission or for processing optical digital data.
It relates more particularly to interferometer structures for use in wavelength converters and add/drop multiplexers used in the telecommunications art. In particular, wavelength converters are used to convert a transmitted optical signal from one wavelength to another wavelength without degrading performance.
2. Description of the Prior Art
Wavelength conversion is used in particular in routing signals to solve contention problems.
In these devices the information is in the form of binary data represented by pulses modulating an optical carrier wave. A binary value is therefore determined by the amplitude (or power) level of the modulated optical wave.
During transmission the signal may be degraded which makes it more difficult to detect high and low levels of the received signal in the receivers.
In the amplitude domain the quality of an optical signal is usually defined by at least two parameters: the signal to noise ratio and the extinction ratio.
The signal to noise ratio is defined as the ratio of the optical power of the signal to the power of the noise in a band of wavelengths containing the wavelength of the signal carrier.
The extinction ratio is defined as the ratio of the powers respectively corresponding to the high and low levels of the signal. This ratio must be sufficiently high despite variations in the input signal.
FIG. 1 shows an interferometer structure in the case of a wavelength converter. It comprises two guide branches 1 and 2. At least one branch includes a semiconductor optical amplifier OA.sub.1. For reasons of symmetry, it is usually preferred to place a second semiconductor optical amplifier OA.sub.2 on the other branch 2. The presence of the second semiconductor optical amplifier OA.sub.2 assures substantially the same Level of amplification in both branches of the structure and consequently substantially identical powers at the output of the interferometer branches.
The two optical amplifiers OA.sub.1, OA.sub.2 are sufficient in themselves to form the interferometer structure when the latter is of the "active-passive" type, i.e. when the guides constituting it are made from two types of material to form active guides and passive guides. In this case the guides at the ends of the interferometer structure form passive guides and are made from a non-absorbing material so that the optical signal at the output from the interferometer structure is identical to the signal from the amplifiers OA.sub.1 and OA.sub.2. However, producing an "active-passive" type integrated structure of the above kind is very complicated because it requires a plurality of successive epitaxial growth steps to deposit the two types of material. Because the fabrication of this structure is slow and difficult, its cost is considerably increased.
To simplify fabrication and to reduce the cost of an integrated interferometer structure of the above kind it is preferable to use an "all active" structure, i.e. a structure in which the guides are all active and are formed of a single material. However, in this case the optical signal from the amplifiers OA.sub.1 and OA.sub.2 is strongly absorbed by the material used and the optical power at the output of the structure becomes much too low to be detected. Consequently, in an "all active" structure it is necessary to add peripheral optical amplifiers to amplify the optical power absorbed and to recover a usable signal at the output of the structure. These peripheral amplifiers are the amplifiers OA.sub.3, OA.sub.4, OA.sub.5 and OA.sub.6 in FIG. 1.
The components of the "all active" interferometer structure shown in FIG. 1 are described in detail below.
A first coupler K.sub.1 couples one end of each of these branches to a peripheral semiconductor optical amplifier (input amplifier) OA.sub.5. A laser 7 supplies to the peripheral amplifier OA.sub.5 an output carrier wave M at wavelength .lambda..sub.o.
A second coupler K.sub.2 couples the other end of the first branch 1 to another input peripheral semiconductor optical amplifier OA.sub.4. The coupler K.sub.2 introduces into the first amplifier OA.sub.1 an input signal I at wavelength .lambda..sub.i that has been amplified by the input amplifier OA.sub.4. The amplifier OA.sub.1 saturates and the state of the interferometer is changed, which phase modulates the output carrier wave.
A third coupler K.sub.3 connected to the coupler K.sub.2, to the second amplifier OA.sub.2 and to another peripheral semiconductor optical amplifier (output amplifier) OA.sub.3 supplies an output signal O resulting from the coupling of auxiliary waves AM.sub.1 and AM.sub.2 respectively supplied by the first and second amplifiers OA.sub.1 and OA.sub.2. The waves AM.sub.1 and AM.sub.2 correspond to the waves M.sub.1 and M.sub.2 from the coupler K.sub.1 and respectively amplified by the amplifiers OA.sub.1 and OA.sub.2. The output signal O at wavelength .lambda..sub.o is then amplified by the output peripheral amplifier OA.sub.3.
Another peripheral amplifier OA.sub.6 is included to preserve the symmetry of the structure and to replace one of the amplifiers OA.sub.3 or OA.sub.4 in the event of a fault.
Respective currents I1 and I2 are injected into the amplifiers OA.sub.1 and OA.sub.2 via electrodes E1 and E2. The output signal O is the result of constructive or destructive interference between the waves AM.sub.1 and AM.sub.2, depending on the phase difference between the two branches of the interferometer.
To assure efficient wavelength conversion the saturation power threshold of the amplifiers OA.sub.1 and OA.sub.2 in branches 1 and 2 of an interferometer structure of the above kind is set relatively low. Consequently, when interference of the waves AM.sub.1 and AM.sub.2 is constructive, i.e. when the waves AM.sub.1, and AM.sub.2 are in phase, the optical powers of the two amplifiers OA.sub.1 and OA.sub.2 add with the result that the optical power in the output amplifier OA.sub.3 is very high.
In this case the output amplifier OA.sub.3 is strongly saturated and the extinction ratio is therefore highly degraded. The gain at high levels becomes lower than the gain at low levels with the result that the output signal O is subject to compression of the high levels and is consequently distorted. This distortion can also occur in the input signal I or in the output carrier wave M. If the input signal I is distorted or if the output carrier wave M is distorted the output signal O is degraded and the extinction ratio of the interferometer structure is reduced.
It is therefore apparent that it is desirable for the peripheral amplifiers to be able to operate under non-saturated conditions.
The input saturation power of an amplifier is generally defined by the input optical power at which the gain of the amplifier is halved.
The drawbacks mentioned above can occur in any "all active" interferometer structure. The aim of the invention is to remedy these drawbacks by proposing a structure which, compared to prior art structures, renders it more difficult for the input power of the peripheral amplifiers to reach the input saturation power. This can be achieved either by increasing the input saturation power of said amplifiers or by reducing the input power of the amplifiers.